Chemical reactors which are based upon dense ion-conducting ceramic membranes are used for processes including oxygen separation, controlled oxidation reactions, and fuel cell applications. For example, mixed ion/electron conducting ceramic membranes may be used to separate oxygen from air and transport oxygen anions to the opposing membrane surface where they can be recombined to form pure oxygen gas or can be used for catalytically-controlled oxidation of feedstock gases to form preferred oxidized product gases. Similarly, fuel cells may be based upon electrically-insulating oxygen anion-conducting or proton-conducting membranes.
Ceramic membrane reactors must be operated at elevated temperature in order to produce ion fluxes of sufficient magnitude for economically viable chemical reaction rates. Reactant gases are sometimes supplied to the ceramic membrane reactor at elevated pressure from prior chemical unit operations. It is also sometimes desirable for reaction products to be supplied at elevated pressure to increase the chemical driving force or to prevent expensive decompression/compression steps.
Operating conditions for ceramic membrane reactors can include pressures up to 600 psi and temperatures up to 1000xc2x0 C. The ceramic membrane is substantially gas-impermeable and typically divides a reactor into an oxidation zone and a reduction zone. The reactor is typically formed by positioning the ceramic membrane and forming a gas-tight seal between the membrane and ceramic or metallic substrates of the reactor.
Practical chemical reactors based upon ceramic membranes must include structures to support the active ceramic membrane between reactant and product gases, to supply reactant gases, to remove product gases, and to isolate reactants from products. Depending upon the details of the reactor, these structures can include various metallic or ceramic membrane holders, manifolds and substrates. These reactors must reliably seal the active ion-conducting ceramic membrane to a holder or substrate in the reactor. Such seals must be gas-tight and able to sustain the operating conditions of the reactor, including the elevated temperatures and pressure gradients, without adversely affecting the active ceramic membrane. The seals must also be physically and chemically stable to provide reliable operation over the lifetime of the membrane, which is typically on the order of one to ten years. The seals must be substantially gas-impermeable and able to withstand exposure to strongly oxidizing and reducing atmospheres, including hydrocarbons, hydrogen, alcohols, H2S, NOx, water, oxygen, and air.
U.S. Pat. No. 2,933,857 relates to semicrystalline, high silica content ceramics with high thermal expansion coefficients which contain (by weight) 85-92% SiO2, 6.5-15% Na2O and/or K2O, 0-8% Al2O3, and 0-5% P where the combined amounts of SiO2, Na2O and K2O are equal to at least 92%. The ceramic material is heated to between 650xc2x0 C. and 1250xc2x0 C. until it has a linear thermal expansion coefficient above 175xc3x9710xe2x88x927 [/xc2x0 C.] between 0xc2x0C.-300xc2x0 C. The ceramic is described as useful for tableware and for seals to high expansion metals and alloys.
U.S. Pat. No. 4,414,282 relates to glass ceramic seals to nickel-based alloys, such as Inconel. The seal is formed using a composition containing (by weight) 65-80% SiO2, 1.5-7% B2O3, 1-8% Al2O3, 1-5% P2O5, 1-8% K2O and 8-16% Li2O. The glass compositions are reported to be sufficiently well-matched to nickel-based alloys to be more resistant to internal cracks than prior art seals. This patent cites several earlier U.S. Pat. Nos. 2,392,314, 3,220,815 and 3,564,587 which relate to glass ceramic seals (SiO2 in combination with Li2O and ZnO or more complex combinations, respectively). A related U.S. Pat. No. 5,820,989 provides a process for preparing a glass ceramic-to-metal seal by thermal processing of a formula very similar to that of the ""282 patent.
U.S. Pat. No. 5,725,218 discloses strontium-borate compositions for use as ceramic membrane-metal alloy sealants in membrane reactors. The sealant composition includes prescribed amounts (about 30-70% by weight) of the metal oxide ceramic membrane material, a fritted compound of Sr, Fe and Co oxides.
The present invention relates generally to glass-ceramic sealants and to methods of making seals between a ceramic material and a substrate which may be another ceramic, metal or a metal alloy. Seals of the present invention employ silicate glass-ceramic materials combined with selected metal oxides and are thermally processed so that the resultant material will have a thermal expansion coefficient that preferably substantially matches or is intermediate between the coefficients of thermal expansion (CTE) of the two materials between which a seal is to be formed. Sealant materials of this invention are formulated to have thermal expansion coefficients under temperatures, pressures and other operating conditions in a given application, to minimize or avoid sealant joint fracture or to extend the lifetime of the joint under harsh operating conditions. Most generally, sealant materials of this invention can be formulated to have thermal expansion coefficients ranging from about 1xc3x9710xe2x88x926 [/xc2x0 C.] to about 25xc3x9710xe2x88x926 [/xc2x0 C.].
The sealants and sealing methods of this invention are particularly useful in forming seals between materials having relatively high thermal expansion coefficients, i.e., between about 5xc3x9710xe2x88x926 [/xc2x0 C.] and about 25xc3x9710xe2x88x926 [/xc2x0 C.] and more useful for forming seals between high thermal expansion materials (having CTE about 8xc3x9710xe2x88x926 [/xc2x0 C.] or higher). The sealants, seals and sealing methods of this invention are generally useful in sealant applications for catalytic ceramic membranes and, more particularly, are useful in fabrication of ceramic membrane chemical reactors.
Preferred silicate glass-ceramic sealant components are selected so that the CTE of the resultant thermally-processed sealant material substantially matches or is intermediate between the CTE of a ceramic membrane and the ceramic, metal or metal alloy substrate to which the membrane is to be sealed. For applications in ceramic membrane reactors, the CTE of the sealant composition is preferably adjusted to substantially match or to be intermediate between the CTE of the ceramic membrane and its supporting substrates under reaction conditions such as: high temperature; the presence of potentially reactive, reducing, or oxidizing gases; potentially high pressures or pressure differentials across the membrane; or the presence of gases that may cause chemical expansion or contraction of the membrane (e.g., the presence of high or low concentrations of oxygen that may cause expansion or contraction, respectively, of the membrane material).
The sealant compositions of this invention may be heat-treated to produce crystalline phases resulting in the physical and chemical properties needed for production of seals having the desired thermal expansion it coefficients. Reliable seals can be obtained if the CTE of the sealant ceramic is intermediate between that of the ceramic membrane and the substrate. In preferred sealant joint assemblies (ceramic-sealant-substrate) the substrate is selected to have a CTE, under desired operating conditions, which is within about 5xc3x9710xe2x88x926 [/xc2x0 C.] of that of the ceramic membrane to which it is to be joined. More preferably, the CTE of the substrate matches that of the ceramic membrane within about 1xc3x9710xe2x88x926 [/xc2x0 C.]. In certain embodiments herein, the sealant is selected so that its CTE is intermediate between the ceramic and the substrate.
Reliable seals can also be attained when the sealant CTE is within about 1xc3x9710xe2x88x926 [/xc2x0 C.] to about 5xc3x9710xe2x88x926 [/xc2x0 C.] of the CTEs of the materials to be joined. Preferred seals are formed when the sealant CTE is within about 1xc3x9710xe2x88x926 [/xc2x0 C.] of the CTEs of the materials to be joined.
Powders of the preferred silica-based sealant compositions are fabricated via conventional glass-making processes into amorphous powders. The amorphous powders may be used directly for seal formation or pressed into appropriate precursor shapes (rings, disks, etc.) to form seals between the ceramic membrane and the ceramic, metal or metal alloy substrate. Sealant powders can also be blended with organic binders to form a paste which is applied between the ceramic and substrate. The joint assembly is then heated to burn out organic binder and subjected to further heat treatment to form the sealant joint.
Sealant materials for use in joining ceramic and substrate materials with relatively high or high CTEs are preferably thermally processed to exhibit relatively high CTEs (under typical ceramic membrane reactor application conditions). Relatively high CTEs are in the range of about 5-25xc3x9710xe2x88x926 [/xc2x0 C.]. These materials are more useful for sealing ceramic membranes and substrates with CTEs in the range of about 1-30xc3x9710xe2x88x926 [/xc2x0 C.]. Formulation and thermal processing of sealant materials for these applications is selected to result in preferential growth of selective high expansion crystalline phases which exhibit relatively high CTEs. High expansion phases may include silica polymorphs of tridymite and cristobalite, as well as lithium silicates or other high-expansion phases.
The invention also includes silica glass ceramic sealant material having CTEs in the range of about 1xc3x9710xe2x88x926 [/xc2x0 C.] to about 5xc3x9710xe2x88x926 [/xc2x0 C.] which are more useful for sealing ceramic membranes and substrates with CTEs in the range of about 0.5xc3x9710xe2x88x926 [/xc2x0 C.] to about 10xc3x9710xe2x88x926 [/xc2x0 C.]. Formulation and thermal processing of sealant materials for this application is selected to result in preferential growth of selective low expansion crystalline phase which exhibit desired CTEs. Low expansion phases include lithium aluminum silicates such as spodumene.
In a preferred embodiment, to further aid in substantially matching the CTE of the seal to that of the membrane, sealants of this invention can optionally comprise up to about 80% by weight of the ceramic membrane material. The ceramic membrane material can be present for an amount from about 0.5% to about 80%. Ranges of ceramic membrane materials added with other sealant ingredients can be from about 25% to about 80% by weight, from about 50% to about 80% and from about 50% to about 75%. Preferred ceramic membrane materials are typically mixed metal oxides which exhibit selective ionic and/or electronic conductivity dependent upon the specific application. For applications in which oxygen anions are transported across the membrane, sealants comprising up to about 80% of an oxygen anion/electron-conducting mixed metal oxide material are preferred.
The CTEs of sealants can be predicted largely by interpolation of the coefficients of the components, weighted according to the volume fraction of each component present. Actual CTEs of a given sealant composition can then be measured by known methods. Preferably the CTE of the sealant is measured by known methods under the conditions of the application for which it is targeted.
In another embodiment, a metal or metallic alloy seal is provided on any exposed surface of a ceramic seal used to bond a ceramic membrane to a substrate or holder. The metal or metallic seal is particularly useful to prevent possible damage to ceramic seals on exposure to reactor atmospheres, e.g., oxygen-containing gases, gases to be oxidized and other potentially detrimental gases, including water vapor, passed into or through the ceramic membrane reactors. Furthermore, the metal or metallic alloy seal, particularly when applied to surfaces of ceramic seals of this invention, can serve to minimize or correct defects in seal porosity, e.g., by sealing cracks in the ceramic seal.